The present invention relates to a method of manufacturing a semiconductor device, especially to a method of manufacturing a semiconductor device that has a deformable thin-layered structure of mono-crystal silicon to be used in a micro-machining or the like.
FIG. 9 illustrates the steps of the conventional method of forming a silicon thin film (Japanese Patent Laid-Open No. 60-121715 (1985)). As a first step, a first silicon substrate 81 provided as a supporting substrate and a second silicon substrate 83 to be thinned are respectively oxidized to form oxide films 82 on the respective bonding surfaces (FIG. 9A).
Then, two substrates are bonded together by performing a hydrophilic treatment on their bonding surfaces. Under such a condition, they are placed into a furnace to entirely bind them together by the heating (FIG. 9B). Subsequently, the second silicon substrate 83 is ground to a predetermined thickness by a grinding machine. Thus, an SOI (Silicon On Insulator) substrate as shown in FIG. 9C is obtained.
On the other surface opposite to a ground surface 84 of this substrate, furthermore, an oxide film is applied as an etching mask. Then, it is etched by hydrazine or other etching solution until a surface of the oxide film 82 is exposed. At the time of exposing the oxide film, the etching is automatically terminated and a silicon thin film is formed.
FIG. 10 illustrates another method of forming a silicon thin film (IEEE ED Vol. 36 (4), p. 663 (1989)). This method is named as an anodic oxidation that utilizes an anodized film as an etching stop. This method is features in the use of a wafer having PN-junction.
A starting substrate to be used comprises a supporting substrate 91 on which an epitaxial layer 92 is grown. The supporting substrate 91 is provided as a silicon substrate having a P-type conductivity, while the epitaxial layer 92 has a N-type conductivity. From the substrate's surface where the P-type silicon is exposed, an etching operation is carried out in an etching solution 93 such as hydrazine through the oxide film 95 as a pattern mask in the presence of positive voltages on the epitaxial layer. While the etching action proceeds and reaches a region of the N-type epitaxial layer, a current flows from the N-type epitaxial layer to the etching solution. As a result, the an oxide film is formed on the silicon's surface. This oxide film prevents the progress of etching, so that the etching is automatically terminated at that position. Consequently, a silicon thin film specified by a thickness of the N-type epitaxial layer 92 is obtained. In the drawing, numerals 90, 94 and 96 are a power supply, an opposite electrode and oxide films, respectively.
However, a grinding technique is used in the method of forming a silicon thin film using the binding process as described above. Therefore, it is difficult to obtain silicon thin films in large quantity with high accuracy without a thickness variation so as to have a thickness of about one micrometer to several tens of micrometers to be required for a micro-machining. In this case, furthermore, the manufactured substrate is very expensive because the necessary amount of silicon corresponds to two wafers and also throughput of the grinding technique is considerably small.
On the other hand, the method of forming a silicon thin film using an anodic oxidation provides a uniform thickness of the manufactured silicon thin film. In this case, however, there is a problem of increasing its manufacturing cost due to the following facts. That is, a specific configuration is required for applying a voltage on a wafer in comparison with the other semiconductor-manufacturing devices. Also, complicated manufacturing steps are required for forming an electrode on the wafer and for difficulties in a realization of the full automatic production.